A free piston Stirling machine typically comprises the components of a cylindrical housing containing a linearly reciprocating power piston and a linearly reciprocating displacer. The piston divides the interior of the housing into two gas spaces. One space is a work space bounded by the piston on the displacer side of the piston, and the other is a back space or bounce space bounded by the other side of the piston. Heat is supplied to the gas in the work space and together with the piston and displacer cooperates to cyclically compress and expand the gas therein to convert a portion of the supplied heat into work. The gas in the back space may act as a spring to limit the motion of the piston during the power stoke of the engine and to sustain the reciprocation of the piston and displacer in timed, although out of phase, relation. Heat is removed from the gas in the work space in an amount equal to the difference in the heat supplied and the work produced as required by the first and second laws of thermodynamics. A regenerator device is employed for regenerating some of the heat supplied from one cycle to the next. The working gas in the engine may be air, hydrogen, helium, other gases, vapors or liquids, or the like.
In a free piston Stirling engine, the power piston may be coupled to magnets which are reciprocated by the piston within alternator windings for converting the mechanical work produced by the engine into electrical energy. A principal advantage of this mode of operation is that the engine requires no external mechanical linkages with other equipment and, therefore, the entire engine may be hermetically sealed. This of course increases reliability and the lifetime of the engine.
While reference is made to a Stirling engine, the invention is equally applicable to other free piston Stirling machines such as heat pumps and refrigerators and to other free piston machines such as a free piston compressor. In such Stirling applications, the direction of the heat and work energy interactions are reversed. The term Stirling machine is intended to refer to Stirling engines, heat pumps, and refrigerators.
Sealing means are provided between the power piston and the inner wall of the housing for substantially sealing the work space from the back space. The sealing means may be in the form of rings or simply a precision fit. In either case, since the piston must be free to reciprocate in the housing, taking into account effects such as thermal expansion, a small annular gap unavoidably exists between the piston and the housing. The working gas may flow between the work and back spaces through the interposed annular gap with the direction of flow being from high to low pressure. The working gas, therefore, will leak from the work space to the back space when the work space pressure is higher than the back space pressure and in the reverse direction when the pressure difference is reversed. The flow through the annular gap is a nonlinear function of the flow rate being proportional to the square of the pressure difference across the gap or some other nonlinear relation.
The back space in Stirling machines is generally designed to have a relatively large volume and, consequently, the pressure of the gas in the back space remains approximately constant during operation. The pressure in the work space, however, undergoes large amplitude changes over the cycle. The work space pressure, when viewed as a function of time, appears as a pressure wave having a series of peaks which rise rapidly above the back space pressure. The peaks are followed by a longer interval of time wherein the work space pressure is slightly below the back space pressure until the next peak occurs. The pressure wave increases and decreases in asymmetric relation to the back space pressure. Although the peaks in the pressure wave last for only a short period of time in relation to the overall period of the cycle, there is a strong potential for gas to leak from the work space to the back space (outward) due to the nonlinearity of the pressure and flow rate relationship. For the interval of time following the peak where the back space pressure is higher than the work space, the flow will be in the opposite direction (inward), however, due to the nonlinear flow the gas flow rate during this interval will be substantially less than during the peak. Over the period of the cycle some of the flow is inward and some of the flow is outward, however, the usual effect of the nonlinearity of the flow is a net transfer of gas from the work space to the back space. This or any other cause of asymmetric leak increases the volume of gas in the back space and decreases the volume of gas in the work space which causes the piston to creep inward from its design mean position.
Imperfections in the piston and housing fit may cause the piston to creep outwardly. However, in a precisely formed engine the nonlinear relation between pressure and flow generally gives rise to an inward creep. Creep in either direction makes it difficult to control the piston position and may degrade engine performance. In other free piston machines a different asymmetrical pressure wave in the work space can also cause inward or outward creep.
Most approaches for automatically centering the piston of a free piston Stirling machine have involved a gas porting system for periodically returning the asymmetrically leaked gas from the back space to the work space. For example, U.S. Pat. No. 4,583,364 teaches a method and apparatus comprising a center port and passageway which is periodically opened by the power piston, thereby permitting a corrective inward flow of gas to balance the outward asymmetric leak. That patent further teaches the use of a displacer having a sealing surface which periodically registers with a port along the passageway to block the outward flow of gas on the piston return (outward) stroke. Prohibiting the flow of gas during the outward stroke improves power output since there will be no gas transfer from the work space to the back space as would otherwise occur.
U.S. Pat. No. 4,404,802 teaches the use of a centering port system which is opened and closed by ports forming a spool valve coming into registry. The work space and back space are brought into fluid communication during both the inward and outward strokes of the piston so that there is gas flow through the centering port system in both directions. In this mode, both inward and outward creep are balanced. That patent does not contemplate a one way centering port. Centering port systems of the type described are generally formed in a portion of the cylinder part of housing and may also include passageways and ports in the piston. They also include a valve linked to the piston which opens at or near the design mean piston operating position. Typically, the valve is formed to interrupt flow in the centering passageway and is a spool valve arrangement in which the piston functions as the spool which covers and uncovers one or more ports in the cylinder or central post to connect the work space and the second space in communication when the piston is near its center position.
Therefore, in summary, in a free piston Stirling cycle machine, an asymmetric pressure wave in the work space causes a problem of keeping the piston centered as a result of a preferential leak from or to the working space to the back space behind the piston. This results in a creep of the average piston position away from its desired center position as a result of a migration of some fluid mass from one space to the other. A conventional method to prevent this inward creep is to allow a communication between work and back spaces through ports in the equivalent of a spool valve mounted on or connected to the piston which come into coincidence at the center of the stroke, so that a flow of fluid can result between the two spaces and allow the average mass in the two spaces to remain fixed and thus the piston to remain centered. This is the accepted way to keep the piston centered, however a power loss penalty accrues from the fact that gas flows both in and out at each passage of the piston both on its in and out passage by the center port.